Height-adjusted whole-body bone mineral content, as measured by dual-energy X-ray absorptiometry, did not change significantly during a year in which 62 adolescents lost averages of 5.5 kg and 2.9 body mass index points, reported Nicolas Stettler, M.D., of Children's Hospital of Philadelphia, and colleagues in the January issue of Obesity.

The researchers also found that participants' height-adjusted bone mineral content score increased in the lumbar spine to 0.40 (SD 0.94) from 0.14 (SD 1.06) at baseline (P<0.001).

The findings should allay earlier fears that obese teenagers who successfully lose weight could suffer major abnormalities in bone development, Dr. Stettler said in an interview.

However, he said there was one finding of concern. Bone mineral content in the arms and legs increased less than expected during the study. The reasons and clinical significance remain uncertain, he said, but deserve additional research. If it is clinically important, he said, it can probably be managed with vitamin D and calcium supplements.

Bone abnormalities during the adolescent growth period could have lifelong consequences, Dr. Stettler and colleagues noted. Some studies have suggested that overweight children and adolescents are at increased risk for fracture. The relationship between obesity, bone mineral content, and fracture risk before adulthood has been unclear.

The new findings came from a study designed primarily to test the effectiveness of sibutramine (Meridia) along with a comprehensive behavioral weight-loss program.

The participants had a mean baseline weight of 100.0 kg (SD 15.3) and age-adjusted body mass index of 36.8 (SD 3.7). They were nine to 17 years old (mean 14.5, SD 1.1).

Dr. Stettler and colleagues found that height-adjusted bone mineral content z-scores were in the 86th to 90th percentiles at baseline for all measures (whole body, lumbar spine, arms, and legs), suggesting high levels of bone mineral content for participants' age and height.

The findings are "important and reassuring," they wrote.

Other explanations for the increased fracture risk seen in obese adolescents should be explored, they added. Perhaps their weight creates greater impacts during falls, or they have decreased coordination and impaired reactions resulting from sedentary lifestyles.

Dr. Stettler said one possible explanation comes from the normal relationship during adolescence between bone growth rates and mechanical loads on the limbs. Weight loss reduces the mechanical load and may therefore scale back the rate of bone mineral accumulation, he said.

Scores remained high for bone mineral content in whole body and lumbar spine measurements as participants lost weight. However, scores for the arms and legs declined slightly, but remained, on average, above the values of the reference group, Dr. Stettler and colleagues reported.

"This is a concern, as most of the increased risk for fractures observed in obese adolescents is due to limb fractures," the researchers wrote, citing earlier studies.

He said the dual-energy X-ray absorptiometry measurements were not the most accurate tool. Additional studies using peripheral quantitative CT imaging will provide better data for understanding the variation in bone mineral accumulation in different body regions seen in the study.

The study was supported by the National Institutes of Health, the General Clinical Research Center of The Children's Hospital of Philadelphia, Knoll Pharmaceutical, and Abbott Laboratories.

No potential conflicts of interest were declared.

In addition to the obese teens in the sibutramine study, the researchers also obtained bone mineral content measurements on 66 primarily normal-weight teens (mean body mass index 21.1) enrolled in an unrelated study.

Reviewed by Zalman S. Agus, MD Emeritus Professor University of Pennsylvania School of Medicine

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